Automatic brightness control for image intensifier tube

ABSTRACT

The brightness of an output display screen of an image intensifier tube is automatically controlled by a currentlimiting means connected between a direct current source and an oscillator. The oscillator supplies an alternating voltage to a voltage multiplier. The voltage multiplier provides accelerating voltages to the individual stages of the image intensifier assembly. The current-limiting means responds to a nonlinear, current sensing element to provide a substantially constant current source at input light intensity levels below a predetermined level, and to provide a substantially unlimited current source when the input light intensity exceeds the predetermined level.

United States Patent 1 Parker et al.

[451 Nov. 12, 1974 AUTOMATIC BRIGHTNESS CONTROL FOR IMAGE INTENSIFIERTUBE [73] Assignee: RCA Corporation, New York, NY.

[22] Filed: Nov. 26, 1973 [21] Appl. No.: 418,920

Related U.S. Application Data [63] Continuation of Ser. No. 346,673,March 30, 1973,

abandoned.

[52] U.S. Cl. 250/213 VT, 313/96 [51] Int. Cl ..H01j31/50, l-lOlj 39/12[58] Field of Search 250/213 VT; 313/108 R,

[56] References Cited UNITED STATES PATENTS 3,345,534 10/1967 Charles250/213 VT CURRENT- REGULATOR 3,711,720 l/l973 Kryder 250/213 VT PrimaryExaminer.lames W. Lawrence Assistant ExaminerT. N. Grigsby Attorney,Agent, or Firm-George .l. Seligsohn; Edward J. Norton [57] ABSTRACT Thebrightness of an output display screen of an image intensifier tube isautomatically controlled by a current-limiting means connected between adirect current source and an oscillator. The oscillator supplies analternating voltage to a voltage multiplier. The voltage multiplierprovides accelerating voltages to the individual stages of the imageintensifier assembly. The current-limiting means responds to anonlinear, current sensing element to provide a substantially constantcurrent source at input light intensity levels below a predeterminedlevel, and to provide a substantially unlimited current source when theinput light intensity exceeds the predetermined level.

14 Claims, 2 Drawing Figures PATENTU, rmv 1 2 I974 CURRENT R OSCILLATORO LTAG E M L H TIPLIER lO-O AUTOMATIC BRIGHTNESS CONTROL FOR IMAGEINTENSIFIER TUBE The invention herein described was made in the courseof or under a contract or subcontract thereunder with the Department ofthe Army.

This is a continuation of application Ser. No. 346,673 filed Mar. 30,1973.

BACKGROUND OF THE INVENTION This invention relates to image intensifiertubes and more particularly to an improved system for controlling thebrightness at the output display screen.

An image intensifier tube is a device in which a visible image isproduced in response to a radiant energy, such as ultra-violet light,visible light or infrared rays. In an image intensifier, the lightreflected from a scene is imaged onto the photocathode of the imageintensifier assembly. The photocathode converts the light image into anelectron image and the electron image is focused and accelerated by avoltage applied to an associated anode of the intensifier assembly. Theelectron image ultimately impinges on a phosphor screen, on which theintensified visible image of the original scene is displayed. Theresulting degree of intensification is dependent on the voltagedifference between the anode and photocathode at each stage of theintensifier assembly.

Presently known image intensifier systems utilize an image intensifiertube, a voltage multiplier for applying a direct voltage between aninput photocathode and an output electrode or screen with voltageconnections to the additional intensifier stages therebetween. Analternating voltage is supplied to the voltage multiplier by anoscillator which is in turn coupled to a direct current source. In orderto control the output display brightness over an extended range of inputlight intensities, various current-limiting means have been insertedbetween the direct current source and the oscillator to limit the directcurrent applied to the oscillator from the current source. As the lightlevel and current demand in these prior art systems increases, a greaterportion of the direct current voltage is dropped across thecurrent-limiting means and a smaller voltage therefore appears at theoscillator input terminal. An example of one prior art system in U.S.Pat. No. 3,71 1,720 which issued to R. A. Kryder and which is assignedto the same assignee as the present invention. Another example of acurrent-limited prior art system is U.S. Pat. No. 3,581,098.

In the operation of these prior art systems, at higher input lightlevels, the output screen illuminance tends to logarithmically decreaseas the input illumination logarithmically increases over the intendedoperating range. This undesired effect results from the currentlimitingaction wherein the available current from the direct current source isinsufficient to supply the stages of the image intensifier tube as thecurrent demand increases.

SUMMARY OF THE INVENTION In an image brightness control system of thetype comprising: an image intensifier tube, a voltage multiplierapplying a direct voltage between the input and output electrodes of thetube, an oscillator circuit for applying an alternating voltage to thevoltage multiplier, a direct current source, and current-limiting meansconnected between the direct current source and the oscillator; theimprovement therewith, comprising: a nonlinear current sensing meanscoupled between the direct current source and an input electrode of theimage intensifier tube for controlling the currentlimiting means toprovide a substantially constant current source at input light intensitylevels below a predetermined level, and to provide a substantiallyunlimited current source when the input light intensity exceeds thepredetermined level.

BRIEF DESCRIPTION OF THE DRAWING:

The advantages of this invention will become more readily appreciated asthe same becomes better understood by reference to the followingdetailed description when taken in conjunction with the accompanyingdrawing wherein:

FIG. 1 is a schematic representation of an image intensifier tube systemincorporating the present invention; and

FIG. 2 is a graphical representation showing the improved responsecharacteristics of an image intensifier tube system in accordance withthe present invention.

DETAILED DESCRIPTION As shown in FIG. 1, an image intensifier system 10includes: a three stage image intensifier tube 12, a voltage multiplier14, an oscillator 16, a current regulator 18 and a direct current sourceor battery 20. A photocathode 12a of image intensifier tube 12 iscoupled to a point of reference potential, schematically represented asground, by way of serially coupled diodes 22a through 22d. Photocathode12a is also coupled to ground by way of capacitor 24. The electricaljunction point between capacitor 24 and diode 22a is coupled to an inputof current regulator 18 by way of lead D. An anode 12a of the firstimage intensifier stage is coupled to a photocathode 12b of the secondimage intensifier stage. An anode 12b of the second stage is coupled tothe photocathode of the third image intensifier stage. Similarly, ananode 12c of the third stage is coupled to a phosphor-coated anode 12dwhich serves as an output display screen.

Stepped accelerating and focusing voltages are supplied to therespective anode and photocathode electrodes between each stage byvoltage multiplier 14. Voltage multiplier 14 comprises a plurality ofcapacitors 26 and diodes 28 arranged in a well-known voltage-doublerconfiguration. A first step connection V of voltage multiplier 14supplies the accelerating voltage for the first image intensifier stage.Similarly, stepped voltage connections V and V provide acceleratingpotentials to the second and third image intensifier stagesrespectively. The current path between voltage multiplier 14 and thefirst image intensifier stage includes a resistor 30.

The oscillator 16 provides an alternating voltage to the voltagemultiplier 14 by way of lead 32. Oscillator 16 may be any one of anumber of known oscillators which convert a direct current input into analternating voltage output. Voltage multiplier 14 in turn converts andmultiplies this alternating voltage up to the direct voltages which areapplied to image intensifier tube 12. By way of example, voltages V Vand V may be 15Kv., 30Kv., and 45Kv. respectively.

The direct current provided by battery 20 is coupled to oscillator 16 byway of current regulator 18. Current from the positive terminal ofbattery 20 flows through the load provided by oscillator 16 and back tothe negative terminal of battery 20 through the collectoremitter path ofan NPN transistor 34. Forward bias for transistor 34 is provided by thebiasing network comprising thermistor element 36a, sensistor element 36band variable resistor 38. This biasing network provides a fixed orconstant current drive to transistor 34 Which in turn acts to limit thecurrent supplied to oscillator 16 to a fixed or constant value.Thermistor element 36a exhibits a negative temperature coefficientwhereas sensistor element 36b exhibits a positive temperaturecoefficient. Accordingly, the values of temperature compensationelements 36a and 36b can be selected so as to track the temperatureversus gain characteristics of transistor 34 in order to maintain aconstant current output over a predetermined range of temperaturevariations. The input D of current regulator 18 is coupled to a secondNPN transistor 40 by way of a variable resistor 42. It can be seen thatthe collector-emitter path of transistor 40 is serially coupled with thepath of transistor 34. Accordingly, when a forward bias is applied tothe base-emitter path of transistor 40, a low impedance path is providedbetween the positive terminal of battery 20 and the base-emitter path oftransistor 34. Under these conditions, transistor 34 goes into a fullyconductive or saturated condition. However, when the bias at the baseinput of transistor 40 is insufficient to cause conduction of transistor40, the conduction of transistor 34 is controlled solely by the biasingnetwork comprising elements 36a and 36b, and variable resistor Theoperation of image intensifier system 10 of FIG. 1 will now be describedin conjunction with the improved response characteristics graphicallyrepresented by FIG. 2. At low light input levels, for example, less than10 foot candles, only negligible current flows in the current path ofthe first image intensifier stage. This path includes serially coupleddiodes 22a through 22d, resistor 30, the portion of voltage multiplier14 which provides accelerating voltage V and the photo-current pathbetween 12a and photocathode 12a of the first image intensifier stage.Thus, only a negligible voltage drop is developed between point D andground. With only a negligible signal voltage being developed at pointD, transistor 40 in current regulator 18 is therefore cut-off.Accordingly, the constant current being supplied by regulator 18 iscontrolled solely by the biasing network comprising elements 36a and36b, and resistor 38.

Thus, at input light levels below 10 foot candles, the direct currentbeing supplied to oscillator 16 is a substantially constant current.Resistor 38 is adjusted to provide a maximum desired brightness in thelight input range of 10 foot candles to 10" foot candles. In currentlypreferred practice, resistor 38 is typically adjusted for a light outputnot to exceed 50 foot lamberts over the above-described light inputrange (i.e. lto foot candles).

At light input levels exceeding l0- foot candles, the first stagephotocurrent is of a sufficie'nt level to cause a significant signalvoltage to be developed across diodes 22a through 22d. As the lightinput increases from 10 foot candles, the signal voltage developedacross the serially coupled diodes increases as a log function of theincrease in input light levels or the corresponding photo-current.Stated differently, the photo-current is directly proportional to thelight input. However, the light input, over the desired range ofoperation, increases several orders of magnitude. Thus, the currentpassing through diodes 22a through 22d also increases several orders ofmagnitude. Hence, advantage is taken of the nonlinear or logarithmiccharacteristics of the diodes wherein the voltage drop across the diodesis proportional to logarithmic changes in current passing therethroughrather than directly proportional to the current changes. It should beappreciated that a linear sensing device such as a resistor woulddevelop a voltage thereacross which would increase linearly over therange of input light levels. Accordingly, under these conditions, thedynamic range of the developed voltage would be excessive, withreference to presently known image intensifier tubes, which thereforecomplicates and is not readily amenable to presently known techniquesfor controlling the associated current-limiting means.

When the signal voltage developed across diodes 22a through 22d issufficient to forward bias the baseemitter junction of transistor 40,transistor 40 begins to conduct and continues to conduct in anincreasing manner as the magnitude of the signal voltage increases. Whenthe signal voltage is sufficient to fully forward bias the base-emitterjunction of transistor 40, transistor 40 conducts heavily therebycausing transistor 34 to become fully conductive or saturated. Oncetransistor 34 saturates in this manner, the currentlimiting action ofcurrent regulator 18 is defeated, and battery 20 supplies the necessaryoperating current through oscillator 16 to maintain a relativelyconstant output display screen luminance. That is, since thephoto-current passing through the individual stages of image intensifiertube 12 increases in proportion to the light input levels, the currentdemand of the image intensifier tube becomes correspondingly great atthe higher light input levels. Hence, in accordance with the presentinvention, the substantially constant current source provided by currentregulator 18 becomes a substantially unlimited current sourcethat is,unlimited relative to the current limited modeand, therefore the currentsource provides a direct current which varies in accordance with thecurrent demand of the image intensifier tube when the input light levelsexceed a predetermined level. In this manner, the current demand of theintensifier tube, at the higher input light levels, is satisfied.

Variable resistor 42 is used as a trimming device to compensate forvarious first stage image intensifier photocathode sensitivities-whichmay vary with different tubes; and to insure that the selected maximumlight output, for example, 50 foot-lamberts, is not exceeded in thelight input range from it) foot candles to 10.0 foot candles. Themaximum input light level range is largely determined by the size(diameter) of the input photocathode electrode and by other physicalconsiderations such as whether an aperture reducing device is insertedbetween the input light source and the input photocathode.

The function of resistor 30 in the first image intensifier stage isexplained in detail in US. Pat. No.

3,711,720. Briefly, resistor 30 controls the voltage applied to thefirst stage in response to the intensity of the input light whichthereby prevents a cut-off condition from occurring when the imageintensifier tube tends to draw excessive current; resistor 30 also actsto imdeveloped across the sensing diodes(s) (s) must be sufficient toovercome the additive base-emitter voltage drops (V,,,,) of thetransistors in the current-limiting means. Further, since the voltagedeveloped across the sensing diode(s) is proportional to a logarithmicfunction of the forward diode current, the current level characteristicsof the particular image intensifier will also dictate the number ofsensing diodes required to provide the desired signal voltage.

While the present invention has been described with reference to apreferred embodiment, it will be understood by those skilled in the artthat other embodiments can be realized. For example, the nonlinearcurrent sensing function can be provided by devices other than thespecific type shown in the drawing. In this regard diodes 22a-22d can bereplaced, for example, by a resistor having a Zener diode poledthereacross. Accordingly, a fixed Zener voltage would be developed atpoint D when the photo-current exceeded a predetermined level. However,this alternative embodiment would be less desirable as the attendanttransistion from the current-limited mode to the unlimited mode would besignificantly more abrupt than the preferred operation depicted in FIG.2.

Further, the function provided by transistor 34 can be provided by aresistive impedance means other than the specific current-limitingsemiconductor device means shown in the drawing. In this regard, aresistor having a fixed given value can be substituted for the resistiveimpedance provided by the collector-emitter path of transistor 34 inFIG. 1. Thus, the collectoremitter path of transistor 40 could becoupled across the resistor in this exemplary alternative embodiment.Accordingly, the effective impedance of the resistor would decrease asthe intensity of the input light exceeded a predetermined level.However, the preferred resistive impedance means is a semiconductordevice arranged to provide current-limiting. That is, as the currentincreases, the voltage drop across a resistor increases; whereas, theeffective resistive impedance of a semiconductor current-limiterincreases as the current demand increases in response to an effectivedecrease of the load resistance. The result is that a resistiveimpedance specifically comprising a semiconductor current-limiterprovides a greater gain or light output level at low input light levels.

What has been taught then is an improved automatic brightness controlfor an image intensifier tube system which maintains a more constantlight output level regardless of the light input level. In currentlypreferred practice, the brightness of the image displayed on the outputscreen is controlled well within a range of output light levels betweenand 50 foot lamberts. It should be appreciated that the output lightlevel variations graphically represented by FIG. 2 are plotted with 6reference to a linear ordinate. It will be appreciated by those skilledin the art, however, that prior art image intensifier tube systemsexhibit a substantially greater output light level variation which isnormally plotted with reference to a logarithmic ordinate.

What is claimed is:

1. In an image brightness control system for an image intensifier tubeof the type comprising: an image intensifier tube having a photocathodeinput electrode emitting electrons in response to the intensity of lightapplied thereto and a phosphor display screen output electrode providingan image display in accordance with said electrons impinging thereon; avoltage multiplier applying a direct voltage between said input andoutput electrodes for accelerating said electrons and directing saidaccelerated electrons to said display screen for increasing thebrightness of said image; an oscillator circuit applying an alternatingvoltage to said voltage multiplier, a direct current source; andcurrentlimiting means coupled between said direct current source andsaid oscillator to limit the direct current supplied to said oscillatorfrom said direct current source to a predetermined current level forlimiting the brightness of said image displayed on said screen; theimprovement therewith comprising:

a nonlinear current sensing means coupled between said photocathodeinput electrode and said direct current source for developing a signalvoltage which is substantially proportional to logarithmic changes ofphoto-current in said image intensifier tube; and

means coupling said current-limiting means to said current sensing meansand responsive to said signal voltage for controlling saidcurrent-limiting means to provide a substantially constant currentsource to said oscillator at input light intensity levels below apredetermined level, and to provide current varying in accordance withthe current demand of said image intensifier tube at input lightintensity levels exceeding said predetermined level.

2. The image intensifier system according to claim 1, wherein saidnonlinear current sensing means comprises at least one diode poledbetween said photocathode input electrode and said voltage multiplier.

3. The image intensifier system according to claim 2, further includinga capacitor coupled across said diode.

4. The image intensifier system according to claim 3, wherein saidcurrent-limiting means includes temperature compensation means tosubstantially maintain said predetermined current level over apredetermined range of temperature variation.

5. The image intensifier system according to claim 1, including aserially coupled resistive impedance coupled between said photocathodeinput electrode and said voltage multiplier.

6. The image intensifier system according to claim 1 wherein:

said tube includes three cascaded image intensifier stages and steppedconnections between said voltage multiplier and said stages, each stageincluding a photocathode input electrode and an output electrode; and

wherein the photo-current path of the first image intensifier stageincludes said current sensing means, a resistive impedance, and theportion of said voltage multiplier which applies a direct voltagebetween said input and output electrodes of said first stage.

7. The image intensifier system according to claim 6 wherein saidpredetermined input light level is substantially equal to 10 footcandles.

8. The image intensifier system according to claim 7, wherein thebrightness of said image displayed on said screen is controlled within arange of output light levels between 20 and 50 foot lamberts.

9. In an automatic image brightness control system for a light imagingdevice of the type comprising an image intensifier tube, a power supplycircuit and means for coupling said power supply to said tube, said tubebeing adapted for operation within the range of input light illuminationlevels from l 'to foot candles;

wherein said tube includes a plurality of cascaded image intensifierstages each stage including a photocathode electrode and an anodeelectrode wherein each anode electrode is coupled to the succeedingphotocathode electrode, the photocathode electrode of the first of saidstages being a photocathode input electrode for emitting electrons inresponse to the intensity of light applied thereto, and the anodeelectrode of the last of said stages including a phosphor display screenoutput electrode providing an image display in accordance with saidelectrode impinging thereon;

wherein said power supply includes a direct current source, anoscillator circuit having an input and an output, a first resistiveimpedance serially coupling said direct current source to said input ofsaid oscillator circuit wherein the alternating voltage at said outputof said oscillator circuit decreases in proportion to the direct currentbeing supplied to said oscillator circuit, and a diode-capacitor voltagemultiplier having an input terminal and a plurality of discrete DC.voltage output terminals, said input terminal of said voltage multiplierbeing cou pled to said outputof said oscillator circuit; and whereinsaid means for coupling said power supply to said tube includes steppedconnections respectively coupling output terminals of said voltagemultiplier to said stages of said tube; the improvement thereincomprising: nonlinear current sensing means coupled between saidphotocathode input electrode and said direct current source fordeveloping a control signal wherein said signal is a substantiallynonlinear function of the photo-current in the first of said stages; andmeans coupling said first resistive impedance to said current sensingmeans and responsive to said control signal for controlling the value ofsaid first impedance, wherein the effective impedance of said firstresistive impedance decreases as the intensity 5 of said input lightexceeds a predetermined level,

thereby to control the output brightness at said screen within apredetermined range of output light levels when said input light exceedssaid predetermined level.

10. The system according to claim 9, wherein said first resistiveimpedance is a resistor having a given fixed value and wherein saidmeans for controlling the value of said first impedance comprises asemiconductor device having first and second main electrodes and acontrol electrode, said main electrodes being coupled across saidresistor and said control electrode being coupled to said currentsensing means.

11. The system according to claim 9, wherein said current sensing meanscomprises at least one diode.

12. The system according to claim 9, wherein said first resistiveimpedance comprises a semiconductor device having first and second mainelectrodes and a control electrode, said main electrodes coupling saiddirect current source to said input of said oscillator, said controlelectrode being coupled to said means for controlling the value of saidfirst impedance, said semiconductor device providing a relativelyconstant current to said oscillator until said input light exceeds saidpredetermined level whereupon the effective resistive impedance of saidsemiconductor device decreases as the intensity of said input lightexceeds said predetermined level.

13. The system according to claim 9, wherein the one of said connectionscoupling a given one of said output terminals of said voltage multiplierto said photocathode input electrode of said tube includes a seriallyconnected second resistive impedance to individually control the voltageapplied to the first of said stages in response to the intensity of thelight being applied to said photocathode input electrode, said secondresistive impedance having a given fixed value which at input lightillumination levels substantially below l0 foot candles results in onlya negligible voltage drop existing across said second resistiveimpedance and at input light illumination levels substantially above l0foot candles results in a voltage drop across said second resistiveimpedance sufiicient to substantially reduce the voltage applied to thefirst of said stages.

14. The system according to claim 9, wherein said predetermined range ofoutput light levels is to 50 foot lamberts.

1. In an image brightness control system for an image intensifier tubeof the type comprising: an image intensifier tube having a photocathodeinput electrode emitting electrons in response to the intensity of lightapplied thereto and a phosphor display screen output electrode providingan image display in accordance with said electrons impinging thereon; avoltage multiplier applying a direct voltage between said input andoutput electrodes for accelerating said electrons and directing saidaccelerated electrons to said display screen for increasing thebrightness of said image; an oscillator circuit applying an alternatingvoltage to said voltage multiplier, a direct current source; andcurrent-limiting means coupled between said direct current source andsaid oscillator to limit the direct current supplied to said oscillatorfrom said direct current source to a predetermined current level forlimiting the brightness of said image displayed on said screeN; theimprovement therewith comprising: a nonlinear current sensing meanscoupled between said photocathode input electrode and said directcurrent source for developing a signal voltage which is substantiallyproportional to logarithmic changes of photo-current in said imageintensifier tube; and means coupling said current-limiting means to saidcurrent sensing means and responsive to said signal voltage forcontrolling said current-limiting means to provide a substantiallyconstant current source to said oscillator at input light intensitylevels below a predetermined level, and to provide current varying inaccordance with the current demand of said image intensifier tube atinput light intensity levels exceeding said predetermined level.
 2. Theimage intensifier system according to claim 1, wherein said nonlinearcurrent sensing means comprises at least one diode poled between saidphotocathode input electrode and said voltage multiplier.
 3. The imageintensifier system according to claim 2, further including a capacitorcoupled across said diode.
 4. The image intensifier system according toclaim 3, wherein said current-limiting means includes temperaturecompensation means to substantially maintain said predetermined currentlevel over a predetermined range of temperature variation.
 5. The imageintensifier system according to claim 1, including a serially coupledresistive impedance coupled between said photocathode input electrodeand said voltage multiplier.
 6. The image intensifier system accordingto claim 1 wherein: said tube includes three cascaded image intensifierstages and stepped connections between said voltage multiplier and saidstages, each stage including a photocathode input electrode and anoutput electrode; and wherein the photo-current path of the first imageintensifier stage includes said current sensing means, a resistiveimpedance, and the portion of said voltage multiplier which applies adirect voltage between said input and output electrodes of said firststage.
 7. The image intensifier system according to claim 6 wherein saidpredetermined input light level is substantially equal to 10 1 footcandles.
 8. The image intensifier system according to claim 7, whereinthe brightness of said image displayed on said screen is controlledwithin a range of output light levels between 20 and 50 foot lamberts.9. In an automatic image brightness control system for a light imagingdevice of the type comprising an image intensifier tube, a power supplycircuit and means for coupling said power supply to said tube, said tubebeing adapted for operation within the range of input light illuminationlevels from 10 5 to 102 foot candles; wherein said tube includes aplurality of cascaded image intensifier stages each stage including aphotocathode electrode and an anode electrode wherein each anodeelectrode is coupled to the succeeding photocathode electrode, thephotocathode electrode of the first of said stages being a photocathodeinput electrode for emitting electrons in response to the intensity oflight applied thereto, and the anode electrode of the last of saidstages including a phosphor display screen output electrode providing animage display in accordance with said electrode impinging thereon;wherein said power supply includes a direct current source, anoscillator circuit having an input and an output, a first resistiveimpedance serially coupling said direct current source to said input ofsaid oscillator circuit wherein the alternating voltage at said outputof said oscillator circuit decreases in proportion to the direct currentbeing supplied to said oscillator circuit, and a diode-capacitor voltagemultiplier having an input terminal and a plurality of discrete D.C.voltage output terminals, said input terminal of said voltage multiplierbeing coupled to said output of said oscillator circuit; and whereinsaid means for Coupling said power supply to said tube includes steppedconnections respectively coupling output terminals of said voltagemultiplier to said stages of said tube; the improvement thereincomprising: a nonlinear current sensing means coupled between saidphotocathode input electrode and said direct current source fordeveloping a control signal wherein said signal is a substantiallynonlinear function of the photo-current in the first of said stages; andmeans coupling said first resistive impedance to said current sensingmeans and responsive to said control signal for controlling the value ofsaid first impedance, wherein the effective impedance of said firstresistive impedance decreases as the intensity of said input lightexceeds a predetermined level, thereby to control the output brightnessat said screen within a predetermined range of output light levels whensaid input light exceeds said predetermined level.
 10. The systemaccording to claim 9, wherein said first resistive impedance is aresistor having a given fixed value and wherein said means forcontrolling the value of said first impedance comprises a semiconductordevice having first and second main electrodes and a control electrode,said main electrodes being coupled across said resistor and said controlelectrode being coupled to said current sensing means.
 11. The systemaccording to claim 9, wherein said current sensing means comprises atleast one diode.
 12. The system according to claim 9, wherein said firstresistive impedance comprises a semiconductor device having first andsecond main electrodes and a control electrode, said main electrodescoupling said direct current source to said input of said oscillator,said control electrode being coupled to said means for controlling thevalue of said first impedance, said semiconductor device providing arelatively constant current to said oscillator until said input lightexceeds said predetermined level whereupon the effective resistiveimpedance of said semiconductor device decreases as the intensity ofsaid input light exceeds said predetermined level.
 13. The systemaccording to claim 9, wherein the one of said connections coupling agiven one of said output terminals of said voltage multiplier to saidphotocathode input electrode of said tube includes a serially connectedsecond resistive impedance to individually control the voltage appliedto the first of said stages in response to the intensity of the lightbeing applied to said photocathode input electrode, said secondresistive impedance having a given fixed value which at input lightillumination levels substantially below 10 2 foot candles results inonly a negligible voltage drop existing across said second resistiveimpedance and at input light illumination levels substantially above 102 foot candles results in a voltage drop across said second resistiveimpedance sufficient to substantially reduce the voltage applied to thefirst of said stages.
 14. The system according to claim 9, wherein saidpredetermined range of output light levels is 20 to 50 foot lamberts.